The conventional Magnetic Resonance Imaging (MRI) scanner cannot be applied in interventional therapy to the extent that the magnet surrounds the patient and the access to the patient is obstructed; meanwhile, a highly uniform magnetic field is required within a sphere of 30˜50 cm diameter and the patient can pass in and out the sphere freely in the conventional MRI scanner, which leads to a large and heavy magnet. A compact and movable MRI system cannot be made.
The above-mentioned two disadvantages of the conventional MRI scanner result from the conventional imaging method. According to the method, the positions of the patient to be examined (sample for short) such as head, thoracic cavity and abdominal cavity etc., must be first entirely placed in a highly uniform magnetic field (the inhomogeneity is within the order of 10−5), which is generally a sphere of 30˜50 cm diameter. Then, by adding the slice selection gradient field, the different tissue slices of the positions to be examined which are entirely in the highly uniform magnetic field, along the direction of the gradient field, are respectively controlled by the magnetic fields of different magnetic intensity. According to the Nuclear Magnetic Resonance Theory, with the radio frequency (RF) field of different temporal frequency, the human body tissue slices can be excited respectively at this time; that is, with the radio frequency (RF) field at the Larmor frequency of the nucleus to be examined corresponding to the magnetic intensity of the set objective slice to be excited, all the spins in the selected objective slice are excited. This progress is called imaging slice selection by the gradient field, or slice selection for short. The slice to be excited is called an objective slice. After the objective slice is excited and the operations to the gradient coils, radio frequency source and spectrometer by the suitable pulse sequence are completed, the magnetic resonance information of the excited slice is obtained. Finally, the magnetic resonance images are obtained by dealing with the information. Repeating all the steps above by changing the frequency of the radio frequency (RF), another selected slice is excited, and the images are obtained following the same steps as the above-mentioned. After all of the images for the set objective slices of the tissues to be examined are obtained, the examination is finished.
The precondition of such imaging method is to require a highly uniform magnetic field in a sphere of 30˜50 cm diameter with the inhomogeneity no more than 10−5 order. To this end, the volume of interest should be in the center of the magnet which surrounds the whole body of the patient. Thus, the magnet is large and heavy and interventional therapy is impossible. A compact and movable MRI system cannot be made.
An imaging method for MRI was presented by the applicant on the basis of a highly open magnet design (referring to the CN patent No. 1371000A). As shown in FIG. 1, the method includes: (1) constructing a volume of interest with a uniform magnetic field in a sheet outside the magnet; (2) moving the objective slice of the tissue to be examined to the position overlapping with the sheet of interest by a servocontrol system; (3) exciting the objective slice with an RF field produced by an RF coil; (4) encoding the spins in the sheet with the gradient field along two orthogonal directions except for the slice thickness direction produced by gradient coils; (5) receiving the signal of the objective slice and reconstructing the image; (6) making another objective slice overlap with the sheet of interest by servocontrol system; (7) repeating the steps from (3) to (6) to obtain the image of another objective slice; (8) repeating steps from (6) to (7) until all the objective slice images are obtained.
A method of fabricating the magnet system is also provided in the disclosure of the prior art. According to the method, a uniform magnetic field in a sheet outside the magnet is obtained and the gradient field along two orthogonal directions except for slice thickness direction is produced in the volume of interest. Referring to FIG. 2, the magnet 10 consists of field yoke 11, the first magnetic stack 12, the second magnetic stack 13, the first gradient coil 15 and the second gradient coil 16. The two magnetic stacks 12/13 are placed on the field yoke 11 symmetrically relative to the field yoke 11. There is an angle between the surfaces of the two magnetic stacks 12/13 facing to the bed. A ferric plate 17 for adjusting the field is placed between the two magnetic stacks 12/13, which can change the distribution of the magnetic field in the area 1 of interest and make the uniformity of the magnetic field meet the requirement. The first gradient coil 15 and the second gradient coil 16 are superposed on the surfaces of the two magnetic stacks 12/13 facing to the bed, respectively.
Furthermore, for the first and second magnetic stacks 12/13, the angle θ between the surfaces facing to the bed is preferably between 0 and 180 degrees. The magnetic stacks are made from many kinds of permanent magnetic materials overlapped in layers, such as permanent magnetic ferrite, alnico, samarium cobalt alloy, sintered NdFeB, forming a wedge form. One side surface of the wedge is attached to the magnetic yoke and another facing to the volume of interest. The magnetization direction of the magnetic material is along the direction from one surface of the wedge to another.
Furthermore, for the first and second magnetic stacks 12/13 made of single magnetic material, the angle between the surfaces facing to the bed is preferably zero. The shape of these two magnetic stacks is strip.
Furthermore, the shape of the ferric plate 17 for adjusting the field is strip shaped. Its cross section is any one of the following shapes: square, rectangle, trapezoid, half circle and circle. The ferric plate 17 is fixed on the joint between the first and second magnetic stacks 12/13 with glue joint or mechanical bonding. If these two magnetic stacks are separated, the slab is fixed on the yoke between these two magnetic stacks.
Furthermore, the first and second gradient coils 15/16 in the magnet system which produce linear gradient field along two orthogonal directions in the volume of interest except for the slice thickness direction are fixed on the surface of the first and second magnetic stacks 12/13 facing to the bed. These two gradient coils with the same shape and number of turns could be overlapped on the magnetic stacks.
Furthermore, in this full open MRI scanner, the bed is made from non-conductive and non-ferromagnetic materials and placed under the volume of interest. The surface of the bed is parallel to the plane of the sheet of interest and lower than it. The altitude of the bed can be adjusted by the servocontrol system manipulating the lift stand on the back of the bed.
According to this technique, the volume of interest is on the one side of the magnet, which permits full access to the patient. This kind of MRI system can be applied in the interventional therapy. In this invention, the volume of interest is not a large sphere but a sheet outside of the magnet, which makes the magnet light and small. The MRI system can be made compact and movable.
In the above-mentioned full open magnetic resonance imaging apparatus, two symmetrical magnetic stacks and a ferric plate for adjusting the field are adopted to form a uniform and parallel magnetic field for the first time. But, the formation and the adjusting method for the magnet are theoretical and not concrete, thus it affects the application of the above-mentioned MRI system.